In the field of semiconductor manufacturing, it has been recognized since the beginning of the industry that removing particles from semiconductor wafers during the manufacturing process is a critical requirement to producing quality profitable wafers. While many different systems and methods have been developed over the years to remove particles from semiconductor wafers, many of these systems and methods are undesirable because they damage the wafers. Thus, the removal of particles from wafers, which is often measured in terms of the particle removal efficiency (“PRE”), must be balanced against the amount of damage caused to the wafers by the cleaning method and/or system. It is therefore desirable for a cleaning method or system to be able to break particles free from the delicate semiconductor wafer without resulting in damage to the devices on the wafer surface.
Existing techniques for freeing the particles from the surface of a semiconductor wafer utilize a combination of chemical and mechanical processes. One typical cleaning chemistry used in the art is standard clean 1 (“SC1”), which is a mixture of ammonium hydroxide, hydrogen peroxide, and water. SC1 oxidizes and etches the surface of the wafer. This etching process, known as undercutting, reduces the physical contact area of the wafer surface to which the particle is bound, thus facilitating ease of removal. However, a mechanical process is still required to actually remove the particle from the wafer surface.
For larger particles and for larger devices, scrubbers have historically been used to physically brush the particle off the surface of the wafer. However, as device sizes shrank in size, scrubbers and other forms of physical cleaning became inadequate because their physical contact with the wafers began to cause catastrophic damage to the smaller/miniaturized devices.
Recently, the application of sonic/acoustic energy to the wafers during chemical processing has replaced physical scrubbing to effectuate particle removal. The sonic energy used in substrate processing is generated via a source of sonic energy, which typically comprises a transducer which is made of piezoelectric crystal. In operation, the transducer is coupled to a power source (i.e. a source of electrical energy). An electrical energy signal (i.e. electricity) is supplied to the transducer. The transducer converts this electrical energy signal into vibrational mechanical energy (i.e. sonic/acoustic energy) which is then transmitted to the substrate(s) being processed. Characteristics of the electrical energy signal supplied to the transducer from the power source dictate the characteristics of the sonic energy generated by the transducer. For example, increasing the frequency and/or power of the electrical energy signal will increase the frequency and/or power of the sonic energy being generated by the transducer.
The relationship between the power level of the sonic energy and particle removal is well known. In essence, higher sonic energy power levels are more effective at removing particles, thus generally resulting in increased PRE. Today, sonic system designs focus on the higher sonic energy power to increase their cleaning effectiveness. Sonic energy has proven to be an effective way to remove particles, but as with any mechanical process, damage is possible and sonic cleaning is faced with the same damage issues as traditional physical cleaning methods and apparatus.
Thus, sonic energy equipment suppliers are constantly trying to balance the desire to achieve high PRE (which is achieved with high power sonic energy) with the desire to minimize damage (which is a side-effect of high power sonic energy). To improve cleaning and to reduce damage caused to wafers by the application of sonic energy, some suppliers have implemented some solutions that control the frequency of the sonic energy, the amplitude of the sonic energy, and/or the angles at which the sonic energy is applied to the wafers. However, even with these controls, damage is still occurring and/or less than optimal PRE is being achieved. Therefore, a need still exists for sonic energy processing equipment and methods that achieve high PRE while minimizing device damage.